Energy-efficient Generating System for HVDC Off-shore Wind Turbine

Licentiatavhandling, 2013

This thesis investigates the design of energy efficient generating systems for off-shore wind turbines. For the generator part of the electric drive system, a surface mounted permanent magnet synchronous generator (SPMSG), an interior permanent magnet generator (IPMSG) as well as a synchronous reluctance generator (SynRG) have been investigated. The system includes a gearbox to increase the rotor speed to be suitable for the generator speed. The system is investigated with IGBT-equipped converters. Furthermore, different dc-link voltages are studied. The results indicate that the most suitable system when cost is ignored includes an IPMSG with a dc-link voltage between the voltage level that gives maximum torque per ampere operation for the whole operation region and the minimum dc-link voltage level which gives the required rated torque. If the IPMSG is replaced by a SynRG with the same size, 74% of the IPMSG rated power can be converted. Furthermore, if instead a SPMSG with the same size is used, the annual energy efficiency of the system is lower compared to the IPMSG, and for both the SynRG and the SPMSG, the power factor becomes lower, compared to the IPMSG. One found disadvantage of the IPMSG is the relatively high torque ripple. Therefore, two methods to reduce the torque ripple are suggested and investigated, showing a substantial torque ripple reduction of about two thirds if either a fractional slot winding or if skewing of the stator is used.